1. Field of the Invention
The present invention relates to a plasma etching method and a plasma etching apparatus for processing an object such as a magnetic film used in magnetoresistive memories, for example.
2. Description of the Related Art
Along with the recent increase of information volume, electronic devices are desired to consume less power, and memories are desired to realize high-speed operation and be nonvolatile. Examples of currently used memories include DRAM (dynamic random access memory) utilizing charge accumulation and flash memory. DRAM is used as the main memory of computers, but it is volatile and loses its memory when power is turned off.
Further, data must be rewritten every given period of time to maintain data during operation, so that it consumes a large amount of power. On the other hand, flash memories are nonvolatile memories, but the writing time of information thereto is slow, which is in the order of μ seconds. Thus, the application of an MRAM (magnetic random access memory) which is a nonvolatile memory that realizes a high-speed operation and low consumption power that overcomes the drawbacks of the prior art memories is expected.
MRAM is a memory that utilizes the change of resistance value due to the orientation of magnetization, and the fabrication thereof adopts a technique related to microfabrication of a magnetic film composed of Fe, Co or Ni formed on a substrate and a nonvolatile metal containing these elements via dry etching using a mask formed by lithography. Further, a similar technique is also adopted in the field of processing magnetic heads using magnetic materials.
Dry etching can be classified into a method using ion beam etching and a method using plasma etching. The plasma etching method is especially adopted widely in the fabrication of semiconductor devices, and has superior productivity since it enables to process large-diameter substrates uniformly. However, if the magnetic film is subjected to microfabrication via plasma etching using halogen-based gas such as F, Cl and Br that had been conventionally used, processing becomes difficult since the vapor pressure of the halogen compounds of the magnetic film is low. Further, since corrosion occurs when the halogen compounds adhered to the magnetic film is exposed to the moisture in the atmosphere, an extra anticorrosion process must be performed.
An etching method using gas containing CO, which includes carbon C and oxygen O as contents, is gaining attention as a method for overcoming the above-mentioned problem. This method utilizes the phenomenon that a metal carbonyl M(CO)x (x being an arbitrary number) with high vapor pressure is produced by the reaction between the metal material M constituting the magnetic film and CO, and the method is suitably applied for processing magnetic films since it does not cause any corrosion.
However, in general, CO is easily dissociated in plasma by the following reactions:CO→C+O2CO→C+CO2 
The drawback of the above method is that CO is reduced by the above-described reaction and metal carbonyl cannot be generated efficiently, and that the C generated by the dissociation is deposited on the magnetic film and inhibits etching.
In order to solve the problem, it becomes necessary to suppress the dissociation of CO in the plasma and generate an active species of CO so as to promote the reaction between the magnetic film and the CO.
Patent document 1 (Japanese patent application laid-open publication No. 2004-356179) discloses a method of creating a high density plasma region by feeding plasma generating gas such as Ar to the upper area of the apparatus, and feeding CO gas to the afterglow region created at the lower area of the apparatus to thereby suppress the dissociation of CO.
Further, non-patent document 1 (Electron-impact dissociation of carbon monoxide (P. C. Cosby, J. Chem. Phys. 98 (10), 1993) and non-patent document 2 (The Theory of Rotating Diatomic Molecules (edited by M. Mizushima (Willy, New York, 1975) respectively disclose reports on the dissociation energy of CO, the vibration excitation energy of CO and the electron excitation energy of CO, and non-patent document 3 (Pramod Subramonium et al.: J. Vac. Sci. Technol. A20(2), 2002) disclose a report on the relationship between the on and off of antenna power and the plasma density.